1. Field of the Invention
This invention relates in general to seismic prospecting, and in particular, it relates to a method for generating seismic signals.
2. Description of the Prior Art
To locate reflecting interfaces in the earth, seismic signals are transmitted through the earth, reflected by subterranean interfaces and are detected and recorded. The time lapse between the transmission, reflection by an interface, and detection of a signal gives the two-way travel time of the signal through the earth, which is then used to locate the interface.
Typically many subterranean interfaces are present in the earth, and a transmitted signal is reflected by a number of such interfaces to produce a number of reflected signals, which are detected and recorded. If the reflected signals do not superimpose significantly when received, the arrival times of the reflected signals may be readily determined to locate the interfaces. Many different methods have been used to generate seismic signals. Where a seismic signal is of short duration, as is the case where dynamite detonation or weight drop is used to generate the seismic signal, the reflected signals do not superimpose significantly and the arrival times of the reflected signals can be readily identified. If a vibratory seismic source is used, the seismic signal generated is of relatively longer duration and the reflected signals from different interfaces typically superimpose one another. The similarity of the reflected signals to the transmitted signal is often masked by superimposition of the many arriving reflected signals. It is difficult to locate, by visual observation of the seismic field record alone, an arrival time at which the reflection from an interface occurs. Correlation methods have been developed to help solve such problems.
The cross-correlation function of the transmitted signal and the received signal is a graph of the similarity between the two signal waveforms as a function of the time shift between them. The waveform of the received signal is obtained from the receiver recording, which typically starts at the time when transmission of the seismic signal begins. In the correlation process, the instantaneous amplitudes of the received signal and of the transmitted signal are multiplied and the product summed over the duration of the transmitted waveform. The process is repeated with the transmitted signal progressively shifted in time relative to the received signal, and the summations are plotted against the time shifts to produce a cross-correlation curve. A typical transmitted signal consists of a sine wave sweeping over a frequency spectrum as a function of time. At the time shift where the similarity in frequency of superimposing parts of the transmitted signal and of the reflected signals is maximum, the sum of the products of the instantaneous amplitudes is higher than those at adjacent time shifts, and the cross-correlation curve has a maximum. The maxima therefore indicate on the recording the arrival times of the corresponding reflected signals. The time shifts from the initiation of transmission to points corresponding to correlation maxima are taken as the travel times through the earth of the sweep signal from the point of transmission to the reflecting interfaces and back to the receiver. For a fuller exposition of principles of correlation, see "Correlation Techniques - A Review" by N. A. Anstey in Volume 12, No. 4 (1964), of Geophysical Prospecting at pages 355-382.
While the cross-correlation method is helpful in revealing the similarity of the detected signal to the transmitted signal, the process of cross-correlation creates certain problems as well. It is well known that where a signal is cross-correlated with itself, in a process known as autocorrelation, there are always side lobes in addition to the principal lobe, or main correlation peak. The side lobes are generally of smaller amplitude than the principal lobe. If the signal used in seismic surveys is crosscorrelated with the received signal from an interface, similar side lobes will also appear in the cross-correlation function in addition to the principal lobe. The principal lobe in both auto-correlation functions and cross-correlation functions for swept frequency signals occurs where the similarity in frequency of the two waveforms correlated is maximum, and therefore occurs at the time shift of interest. In the cross-correlation method the time shift at which the principal lobe occurs indicates the time of arrival of the reflected signal.
Frequently there are more than one subterranean interface that reflect the transmitted signal, and consequently more than one correlation maximum or principal lobe will then appear in the cross-correlation function. The interfaces are usually at different depths and the reflected signals from such interfaces are usually weaker for deeper interfaces. Also, some interfaces do not reflect as much of the signal as other interfaces. The principal lobes corresponding to the interfaces therefore have different amplitudes. It is important to be able to identify in cross-correlation functions such principal lobes that correspond to subterranean interfaces. The side lobes that are inherent to the correlation process, even though of lesser amplitudes than the principal lobe, may be mistaken as the principal lobes corresponding to other interfaces. To avoid such mistakes, it is important to reduce the side lobes that accompany the principal lobe in cross-correlation functions.
It is generally acknowledged that methods of reducing autocorrelation side lobes of a seismic signal will also reduce similar side lobes in the cross-correlation function of that seismic signal.
In the past seismic signals have normally been generated by transmitting from each seismic source a swept sine wave over the entire selected frequency range of interest. It is difficult, however, to manufacture seismic sources which operate efficiently over the entire range of interest which may be from 10 to 160 Hz, or more. To improve transmission efficiency, it may be preferable to utilize a plurality of sources each adapted to operate efficiently over a portion of the desired frequency range, to generate simultaneously portions of the frequency range of the desired signal. Alternatively, it may be desirable to utilize a single vibrator, or a plurality of vibrators, to successively generate portions of the selected swept sine wave seismic signal.
Methods of transmitting seismic signals have been developed wherein different portions of the selected swept sine wave seismic signal are transmitted, successively in some methods and simultaneously in others. It has not been known, however, how to generate a selected swept sine wave seismic signal by sumultaneously generating a plurality of component signals covering different portions of a desired frequency spectrum without generating high amplitude side lobes within the correlation function. Nor has it been known how to generate a swept sine wave seismic signal by transmitting successively a plurality of component signals covering different portions of the desired frequency spectrum, and stacking the reflected signals prior to correlation without generating such high amplitude side lobes.
Margin, in U.S. Pat. No. 4,037,190 (1977), proposed using a number of sweeps having different beginning and final frequencies successively generated by the same vibrator, and stacking the correlograms corresponding to these sweeps. Martin claimed that, since the sweep signals have different frequency ranges, the side lobes from the cross-correlation function of each sweep signal will be dissimilar. By stacking a sufficient number of such cross-correlation functions or correlograms, Martin claimed that the side lobes will tend to cancel one another, while the main lobes will simply add.
Using signals covering different frequency ranges may or may not produce correlograms with side lobes that are dissimilar. Depending on the particular frequency ranges selected, the side lobes may be dissimilar, or they may actually be quite similar so that stacking the correlograms will yield strong side lobes. Martin's method also requires the repetition of the steps of transmitting a sweep signal, detecting its reflection, and cross correlating for a large number of sweeps. Such steps are repeated successively with different sweep signals. Considerable field time is therefore required.
Werner et al, in German Offenlegungschrift No. 2,728,373, laid open on July 20, 1978, disclosed a method for improving correlation where one vibrational source is used. The transmitted signal is composed of several component signals that are transmitted consecutively. One disclosed transmitted signal, which contains alternating upsweep and downsweep component signals, is stated to be suitable for reducing certain interferences due to harmonics by improving the symmetry of the auto-correlation pulse. Two signals were given by Werner et al as examples to illustrate this claim. However, if the several component signals are first stacked and then auto-correlated, the auto-correlation functions produced by using these two illustrative signals will still contain strong side lobes.
The first of the two illustrative signals described by Werner et al comprises four component signals generated consecutively: an upsweep from 12 to 80 Hz, a downsweep from 80 to 12 Hz, another upsweep from 12 to 80 Hz, and another downsweep from 80 to 12 Hz. The four component signals contain exactly the same frequency content. The four signals are stacked and then auto-correlated in order to obtain the auto-correlation function of the composite signal. Thus when a portion of one of the component signals superimposes a portion of a different component signal in the autocorrelation process wherein the two portions have similar frequencies, the auto-correlation function will have a higher value at that particular time shift, thereby producing a strong side lobe. Since the four component signals have exactly the same frequency content, the superimposition of portions of different component signals with similar frequencies will occur frequently in the auto-correlation process. The auto-correlation function of the first illustrative signal will therefore contain a number of strong side lobes.
The second illustrative signal in Werner et al comprises four component signals generated consecutively: an upsweep from 12 to 80 Hz, a downsweep from 76 to 16 Hz, another upsweep from 21 to 71 Hz, and another downsweep from 64 to 27 Hz. While the four component signals do not contain exactly the same frequency content, the four component signals contain the frequencies from 27 to 64 Hz, which comprise a major portion of the frequency content of each of the four component signals. If the four signals are stacked and then auto-correlated, the superimposition of portions of different component signals with similar frequencies will also occur frequently in the auto-correlation process. Therefore, the auto-correlation function of the second illustrative signal will also contain a number of strong side lobes.
A method of simultaneous transmission by several vibrators to survey cross-dip components of subterranean interfaces was disclosed by Anstey et al in U.S. Pat. No. 3,885,225 (1975), based on an earlier United Kingdom application filed on July 21, 1972 (34164/72). The vibrators are spaced apart transversely of the line of profile. The normal frequency bandwidth of each vibrator is divided into several parts, and these are allocated to individual vibrators. Mutually exclusive parts of the bandwidth are emitted by the several vibrators at any one time. The reflected signals originating from the several vibrators are recorded on a single recording. The recorded signals are cross-correlated with each of the signals covering different parts of the bandwidth so as to separate reflected signals from different vibrators on the basis of frequency. Three dimensional information of the interfaces is thereby provided. Therefore, instead of correlating the recorded signals with the composite signal formed by combining all the simultaneously transmitted signals, Anstey et al proposed correlating the recorded signals with each of the simultaneously transmitted signals separately to yield three dimensional subterranean information.